High-impact polystyrene (HIPS) resins are generally produced by subjecting a styrene monomer to bulk polymerization, solution polymerization or bulk-suspension polymerization in the presence of any of various kinds of unvulcanized rubber and have a structure that rubber particles are dispersed in a matrix of a polystyrene resin, whereby the impact resistance of the polystyrene resin, which is rigid and brittle, is markedly improved. Since the high-impact polystyrene resins are cheap and excellent in processability and various physical properties, they have been used in wide application fields.
Polybutadiene rubber and styrene-butadiene copolymer rubber are generally used as the unvulcanized rubber for the high-impact polystyrene resins. In the case where high impact resistance at a low temperature is particularly required, various kinds of polybutadiene rubber are in use. More specifically, for example, the so-called low-cis-polybutadiene rubber obtained by anionic polymerization making use of a catalyst composed of an organolithium alone or comprising the organolithium as a main component, or the so-called high-cis-polybutadiene rubber obtained by using an anionic coordination catalyst comprising a compound of a transition metal such as cobalt, nickel or titanium as a main component is in use.
In recent years, the high-impact polystyrene resins have shown a tendency to be used in wider application fields because of their good various physical properties and processability. As the high-impact polystyrene resins are used in wider application fields, the performance characteristics required of the resins become higher than before. With respect to the physical properties, there is a demand for improvements in balance between impact resistance and stiffness (modulus in flexure), low-temperature impact resistance, and appearance such as gloss. The high-impact polystyrene resins obtained by polymerizing a styrene monomer in the presence of unvulcanized rubber are also required not to deteriorate their physical properties or scarcely deteriorate the physical properties even when the resins are used after they are further diluted with a polystyrene resin or the like or compounded with various kinds of additives such as a flame retardant.
As a method for more improving the low-temperature impact resistance, it has heretofore been proposed in Japanese Patent Publication No. 14689/1992 to prepare polybutadiene rubber having an extremely low 1,2-vinyl bond content and a narrow molecular weight distribution using a special catalyst comprising a rare earth metal compound as a main component and use the polybutadiene rubber thus obtained as an impact modifier. According to this method, a high-impact polystyrene resin excellent in resistance to heat deterioration and weather resistance can be obtained. However, its improving effect on low-temperature impact resistance is not yet sufficient, and its improving effects on stiffness and impact resistance are also not satisfactory.
Japanese Patent Publication No. 5789/1995 has proposed a method of producing a high-impact polystyrene resin improved in impact resistance and gloss, in which a blend of polybutadiene rubber having a high intrinsic viscosity and polybutadiene rubber having a somewhat lower intrinsic viscosity is used as an impact modifier. According to this method, the improving effects on impact resistance and gloss can be brought about. However, since the molecular weight of a low molecular weight component in the blend is high, and the molecular weight distribution thereof is narrow, the impact resistance of the resulting polystyrene resin is not sufficient, and the impact resistance is more markedly deteriorated when the resin is used after diluted with a polystyrene resin or the like or compounded with various kinds of additives.